How can I switch the direction of a Curve on Blender 2.8?
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right click > Switch Direction

(or   right click > D)


There isn't a more direct shortcut, but these are pretty quick! However, if you need to use this option so often that you'd like to only use the keyboard, you can Right click on the "Switch Direction" menu option ad do "Add shortcut". 

By design, in Blender 2.8x, the buttons F5 to F8 are free for add-ons and users to use with their favorite shortcuts.